Method of treating coiled tungsten filaments



Ap 1948. J. A. M. VAN LIEMPT 2,439,913

METHOD OF TREATING COILED TUNGSTEN FILAMENTS Filed Feb. 26, 1945 InvenTon: Johannes A.l\/1.vc:n Liennpl',

His ACT Tonnes.

Patented Apr. 20, 1948 aaunirsio stars-s e NT G' F FIGE .-.-ME';EHOD- OF' TREATIN G COILED TUNGSTEN FILAMENTS "Johannes Antonius Maria van Liempt, :Eindhoven,

Netherlands; as'signor to General Electric"Gom- :.:.pany,. a corporation of New- York Application February 26, 19.43,. Serial No.'.47.7,326 In the Netherlands October 28', 1341 4fGlaims. 1

i In the=incandescent=lamp industrwit has long been theaim to manufacture tungsten 'wire -glow-sbodies'- orfilaments -consist-ing, of: large overlapping crystals. This is more important in so-called spiral or cofled filament;lamps, in which the filament-is 'wounctin ahelicalxform, and= especially-When *the=filament is helically wound several times -(-twice or more) a into a --coiled=coil, as it -is-=broa'd1y-termed. For it has been found that-in such-lamps the sagging ofthe filamentand the consequent light -reduction' during 1 the life of the-lamp is considerably less when the filamentconsists enlarge crystals which generally com-prise one-ormore turnsor convolutions of asingle -coi1'or, in the case-of acofled-coil; one or more turns-of the first heli- 'cal"coiling tharr when the filament-shows a fine crystalline structure.

'-Besides+sagging; :1 the amount 01 vibration-resistance orviendurance is of greatimportance in appraising- 'the qualityof a filament. Toascertain3'thelvibration-resistance or life under-"vibration, none 2 may i-proceed' =-in f the following I way.

Lamps with filaments whose vibration endurance is to be determined-- areburnedintermittently while: on a.'vibration burningdrame; an hour at "rated voltage :and am-hour switched ofi in alternation. :The-v vibratiori-hurningfirame "vibrates, 'ioriexamplegat a 'frequencyofiwto50r'vibrations perrsecond, and vvith"aver-ysmallamplitude e. g.. 0.2mm. *1 Such Vibrations: correspond witli 'those to which incandescent lamps areoften exposed in normal service, as in the cpresencaiof-motors and =:otherzmachinery.

In themselves; 'ifilaments witm high vibrationendurance are; :already known. As employed in xstraightzor iplain'twirecvacuum lamps; for example,

:they :show. a very-fine structure, because small amounts: of rthorium' oxide 2 have :been added (to v the tungsten. .iHOiWIGVEE; such material cannot be .usedsfor making coiled 7.012: coiled-coil filaments, because =1 it :sags badly in: service. Carbon filalment lamps al'soshow good 'lile under vibration;

xhut thesetlampshave only low 'efflciency, and a carbon" filament-cannot be helically "coiled; speakingin a=practical=sense withthe materials :and' methods hitherto used it was not reallypossible-to make acoiled :had i highvibration-endurance.

""I'he inve'ntor haswnowLdiscover-ed that the vibration-endurance of a filament does-hot depend so muchpn'the size of the crystalsof which it is composed; or =on ther WELYE in: which: the adj'acent crystals a-re associated or interengaged in thefilament, *as on-*thephysical character of the internal structure of the individual crystals. *I-Ie has further found that vibration-resistant "this .'difierence"in extensibility, vibration-resistanVfilaments according to the invention show. when etched practically the. same structurepunder microscopic examination, as the 'old coiledlfilaments that do-not i'stand vibration.

'i'Itjs true lthatlthe known coiled. single-crystal wires are .extensiblewhen in anjunused lamp; but; they lose; this ,propertyjn' use. During the coiling, itheyphave undergone .a, .process .of

' deformation, andithey mill eventually recrystallize While in operation.

. As aresiult oi his.disooveryifthatithe vibrationendurance of a filamenttdepends on. the physical character of. the internal structure of. theindi- .vidual.;crystals, ..the; inyentor; "has" .found that vibration-resistant and 7, very extensible. single coil .or.,coi1ed-.coi1recrystallized filaments. can be made ,of ltungstem .wlrehwhichi naturally. .gives a recrystallized structure .'..consls.ting of .large crystals v that haveuat. least. about. :an average crystal size of one turn or convolution of a single coil, or of=the firstrcoiling in the case of a coiled-coil. It .is.. des,irab1 further or t t the wire "materialshould contain traces .of iron,

..for example, 0.05 to..0.. llpencentby. weight, since thus. the application 10f lthe method or process in accordance-with the-invention results in greater :v-ibration-enduranceof thefilaments. 1 The ex- --tensible vibrations-resistant coiled filaments according totherzinyenti-onare.;useful'for the. elecrtrode-oncathode coilssof electric discharge devices or tubes, including discharge lamps and especially fluorescent lamps of the ordinary lowpressure positive column .type, as well as for the glow-bodies of incandescent lamps. Both inc ndescent lamps --and- -idischarge' devices are hereinafter referred to generally and broadly as electric lamps.

The method or process employed may start with coiled filaments without mandrels, which have already undergone the usual setting heat treatment for the relief of deformation-stress (in the case of single coils, on iron mandrels, in the case of coiled-coils, on molybdenum mandrels, for example), and from which the mandrels used in the coiling have been removed, as by a chemical method. At this stage, and before being exposed to a temperature of recrystallization or graingrowth, the filaments are preliminarily subjected to heat treatment between about 1500 C. and 2000 C., with a heating period of at least about 5 minutes. As a result of this, a considerably advanced lattice-restoration or readjustment occurs amongst the filament atoms or molecules, and this readjustment conduces to the desired characteristics of the filaments as regards extensibility and vibration-endurance. To produce this advanced lattice-readjustment, the coils may be heated in a tungsten furnace (for instance) for a period of 5 minutes to two hours, at a temperature of about 1850 C. Naturally, this should be done under protective conditions that insure against any contamination of the tungsten metal, as in an atmosphere of pure, dry hydrogen. The operation can alsobe carried out in a lamp (in which the filament has been mounted as new al) by aging this lamp (e. g., a 130 volt lamp) for 5 minutes to two hours at an under-voltage (e. g., 70 volts) corresponding to a temperature of 1850 C. Speaking in a general way, the coils should be heated up to the highest possible temperature at which in 5 to 120 minutes recrystallization or grain-growth just does not occur.

After initial treatment at about 1850* 0. outside of a lamp, the coils may be fully recrystallized in a tungsten furnace at about 2400 C. (under protective conditions as indicated above), and then mounted in a lamp; or they may be mounted in a lamp first and then burned or aged. For the sake of greater vibration-endurance, the latter method is best, because thus the portions of the filaments in and near the eyelets of the supportwires retain the structure which they acquired during the heating at 1850 C., and do not recrystallize fully; and so such lamps are more re- T(13.5+log t) =0 where T is the absolute temperature and t is the time of heating in seconds, while 0 as usual denotes a constant. The value of C depends on the degree of deformation that the metal crystals have undergone, and the kind of tungsten wire. A corresponding curve I may be drawn with the 'values of log t as abscissas and the values of T as ordinates.

It is obvious that this curve I crosses the T axis at a point F whose ordinate is In accordance with this invention, thelatticereadjustment is accomplished in the right-hand.

4 part of the diagram between 1500" C. and 2000 0., with a heating time of 5 minutes to 2 hours; and as the curve I represents lattice-readjustment, so a certain portion (AE, say) of this curve I defines the range that is used according to the invention. This is in contrast with what happens in the case of an ordinary incandescent lamp, where both lattice-readjustment and recrystallization occur (more or less concurrently) at the operating temperature of the lamp or higher: that is, the heating is considerably above the range of mere lattice-readjustment; and in the diagram, it would be above the aforesaid curve I, generally in the neighborhood of another curve II which expresses the relation between T and log t for recrystallization. Pursuant to the invention, on the other hand, one heats only according to a lattice-readjustment value of C which is 5 per cent to 20 per cent smaller than the recrystallization constant for a heating time of atleast 5 minutes, where the recrystallization constant is understood to be equal to the product T(13.5+log t) when the heating time t in seconds and the temperature T are so chosen that recrystallization just does take place.

Although the crystal size is affected but little, if at all, by the described method of producing vibration-resistant filaments, very surprisingly the extensibility of the coiled filaments produced according to the invention is much greater than that of filaments manufactured in the usual way. By the method or process according to invention, simple coils can be made which in the recrystallized state can be elongated fivefold, and oncecoiled-coils (double-coils) which in that state can be elongated ten to fifteenfold. On the other hand, the extensibility of coils made in the usual way is no more, as a rule. than such that they can be stretched out to double their length. It may be that the greater extensibility of coils according to the inventio is a consequence of the fact that they show far fewer crystal faults and spots of irregular current distribution than th coils manufactured in the usual way.

Besides the advantage of greater vibration-endurance, very extensible coils present still other advantages. They can, for. example, be mounted by machinery with relative ease. They also offer advantages in cases where a filament is stretched out in mounting, as in the manufacture of long tubular incandescent lamps, such as stage or show-window lamps.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of treating a coiled tungsten filament wire which when recrystallized consists of crystals of an average size of at least one wire turn, and is thus resistant to sagging, to give the coil high vibration-endurance and high extensibility in the direction of its axis at ordinary room temperature, as contrasted with poor corresponding characteristics without this treatment; which method consists in maintaining the coiled wire at lattice-readjusting temperature until lattice-restoration has been effected before allowing the wire to reach a higher temperature that results in recrystallization.

2, A method of treating a coiled tungsten filament wire which when recrystallized consists of crystals of an average size of at least one wire turn, and is thus resistant to sagging, to give the coil high vibration-endurance and high extensibility in the direction of its axis at ordinary room temperature, as contrasted with poor corresponding characteristics without this treatment; which method consists in subjecting a coiled tungsten wire of this character containing traces of iron of the order of 0.05 to 0.1 per cent by weight to lattice-readjusting heat between 1500 C. and 2000 C. for at least five minutes, and'until lattice-restoration has been effected, before subjecting the wire to a higher temperature.

3. A method of forming high vibration-resistant and highl extensible coiled tungsten filament wire, which consists in coiling on a mandrel a tungsten wire which when recrystallized consists of crystals of an average size of at least one wire turn, annealing the mandrel-supported coiled wire to eliminate the deformation strains therein, removing the mandrel from the coiled wire, and then subjecting the coiled wire to a temperature between 1500 C, and 2000 C. for a period of at least five minutes to effect a far advanced lattice-restoration thereof, before subjecting the said coiled wire to a higher temperature at which recrystallization thereof would ocour.

4. The method of providing an electric incandescent lamp with a high vibration-resistant coiled tungsten filament, which method consists in coiling a tungsten wire which when recrystallized consists of crystals of an average size of at least one wire turn, annealing the coiled wire to eliminate the deformation strains therein, mounting a REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,280,704 Fuller Oct. 8, 1918 1,815,779 Karef et a1 July 21, 1931 1,826,524 Ramage Oct. 6, 1931 2,114,426 Laise Apr, 19, 1938 2,165,310 Spaeth July 11, 1939 FOREIGN PATENTS Number Country Date 267,312 Great Britain Mar. 17, 1927 468,449 Great Britain July 6, 1937 

